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What is a white dwarf?

Stars go through a number of changes as they pass through different life stages. If a star doesn’t have more mass than about eight times that of our own Sun, then it will end its life as a White Dwarf. White dwarfs are extremely small, hot and compact objects, composed of degenerate matter, and are all that remains after the original star runs out of fissionable fuel and ejects its outer layers into space. What remains is usually around the same mass as the Sun, but is compressed into a ball about the size of the Earth. Such a high density means that one liter of degenerate matter from a white dwarf (a cube with a height of 10cm, or about 4 inches), would weigh one thousand tonnes. Despite their enormous density, white dwarf stars are not able to sustain any fusion reactions, so this is the last dying stage of a star’s lifecycle. They shine bright white because they still carry a lot of heat from back when they were burning fuel in their cores, but over time they will gradually cool, and dim to yellow, then red, and finally go dark altogether. We have never seen a white dwarf cooled down to this degree, however, as it takes a very long time to radiate away that much heat – many times longer than the current age of the Universe!

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Allen is an amateur astronomer, an IT professional, a podcaster, a father of five beautiful kids, a barely competent chess player, and the owner and publisher of the Urban Astronomer podcast and website.
He is also the director of the Citizen Science Section of the Astronomical Society of South Africa, where he tries to help ASSA members do good work with citizen science projects.